The present invention relates to a semiconductor laser module for emitting a laser beam, and more specifically to a semiconductor laser module for an erbium doped fiber amplifier (EDFA) pumping purpose.
Conventionally, in order to stabilize the wavelength of light emitted from an emitting device, optical feedback method is generally used such that the light emitted from a multimode emission laser is partially reflected back to the laser thereby make the emission wavelength of the laser constant.
For examples, a semiconductor distributed feed back (DFB) laser, a distributed bragg reflector (DBR) laser, etc. belong to this classification. The DFB laser is a laser in which a diffraction grating is formed in an active layer of a semiconductor laser device. The DBR laser is a laser in which a reflecting diffraction grating which is, unlike an active layer, transparent in connection with emitted light is formed in a semiconductor laser device along its waveguide portion made of a semi-conducting medium, so that light may be reflected (fed back) to an active layer.
Further, in recent years, a fiber bragg grating, that is, an optical fiber whose core has a refractive index varied in its axial direction so that the optical fiber may have a function of optical diffraction is developing rapidly, and lasers using various fiber bragg gratings are already disclosed. For example, Bulletin No. OPE97-1 of Society of Electronic Information and Communication discloses a xe2x80x9cfiber grating external resonator type multiwavelength laser arrayxe2x80x9d by Kato et al., and Bulletin No. OPE97-2 thereof discloses a xe2x80x9cUV induced waveguide grating and application thereof to an integrated external resonator type laserxe2x80x9d by Tanaka et al.
Further, Atsushi Hamakawa et al. have reported on a technique of using an FBG to stabilize the emission wavelength of a 1480 nm-band pump laser which is to be used as a light source for EDFA pumping, in the 2nd Optoelectronics and Communications Conference (OECC ""97), Technical Digest, July 1997, Seoul, Korea (classification 9D2-5, pages 224 to 225).
However, the above mentioned emitting devices such as the DFB laser and the DBR laser have a single mode emission spectrum. They are used exclusively for communication and not suitable for amplifying an EDFA.
Further, the above mentioned external resonator type lasers disclosed by Kato et al. and Tanaka et al. have the following problems.
1) Those external resonator type lasers are signal mode emission lasers, and mode hop, that is, a shift of central wavelength of emission happens when operating temperature changes by several degrees [C]. Thus, the stability of emission wavelength under change of temperature is low.
2) In those external resonator type lasers, the distance between an emitting surface of a laser device and a diffraction grating for optical feedback, which constitute an external resonator, is short. The process of assembling a module is therefore troublesome and needs special means such as a lensed fiber, flat mounting and the like. In other words, it is difficult to form the above mentioned external resonator type lasers using a two-lens compound confocal coupling system as used in an ordinary Fabry-Perot laser diode or an ordinary pump laser diode.
3) Output power of those external resonator type lasers is low. Therefore, they are not suited to be a light source for exciting an EDFA.
Further, the above mentioned technique on which Atsushi Hamakawa et al. have reported has the following problems though it is suitable for exciting an EDFA:
1) It needs to use, as a diffraction grating, a special fiber bragg grating capable of reflecting light of two different wavelengths.
2) Mode hop due to change of operating temperature is relatively small, but there still happens mode hop of about 2.6 nm.
The present invention has been made in consideration of the above problems. The object of the present invention is to provide a semiconductor laser module which has high power, shows high stability of emission wavelength under change of temperature, and is suited to be a light source for exciting an EDFA.
In order to attain the above object, the present invention provides a semiconductor laser module comprising a semiconductor laser device having an emitting surface from which excited light is emitted and a reflecting surface opposite to the emitting surface, and an optical feedback medium for feeding most of optical power emitted from the emitting surface of the semiconductor laser device back to the semiconductor laser device and emitting part of the optical power, wherein the semiconductor laser device has a first multilayer coating formed on the emitting surface and having a reflectance of 10xe2x88x924 to 10% at the central wavelength of reflection, and the first multilayer coating has a reflection spectrum in the form of a curve having minimum values on both sides of the central wavelength of reflection.
In this semiconductor laser module according to the present invention, mode hop, that is, a shift of central wavelength of emission, caused by operating temperature of the semiconductor laser device itself is restrained.
Specifically, in an ordinary laser, light is reflected using a reflection reducing coating which does not have wavelength selectivity and has a flat reflection spectrum. For example, in a 1480 nm-band multimode emission semiconductor laser module, when the operating temperature changes within the range of 5 to 65xc2x0 C., the central wavelength of emission spectrum shifts to the extent of about 30 nm at most.
In contrast thereto, in the semiconductor laser module of the present invention, the semiconductor laser device has, for example, a dielectric multilayer coating formed on its emission surface, and the dielectric multilayer coating has a reflection spectrum in the form of a curve having minimum values on both sides of the central wavelength of reflection and has a reflectance of 10xe2x88x924 to 10% at the central wavelength of reflection. Therefore, in the semiconductor laser module of the present invention, the extent of a shift of central wavelength of emission spectrum of the semiconductor device under change of operating temperature is restrained to be small, that is, several nanometers at most.
It is desirable that the first multilayer coating has a reflection spectrum which has a maximum value near the central wavelength.
Further, it is desirable that the optical feedback medium is an optical waveguide or an optical fiber which has a core, a diffraction grating is formed in the core along the optical axis thereof, and the optical feedback medium is arranged to face the semiconductor laser device leaving a space of at least 10 mm as measured from the diffraction grating to the emitting surface of the semiconductor laser device.
Further, it is desirable that the semiconductor laser device has a second multilayer coating formed on its reflecting surface, and the second multilayer coating does not have wavelength selectivity and has a reflectance of 90 to 98%, so that the semiconductor laser device may have a multimode emission spectrum.
Due to the above described features imparted by the present invention, the semiconductor laser module of the present invention has high power, shows higher stability of emission wavelength under change of temperature, and is suited to be a light source for EDFA pumping.
Here, an example will be taken in which the optical feedback medium is arranged to face the semiconductor device leaving a space of at least 10 mm as measured from the diffraction grating to the emitting surface of the semiconductor device. The semiconductor laser device has a multimode emission spectrum, and its device length is 800 xcexcm. When the semiconductor laser module operates in a compound resonance mode, the compound resonator length is 800 xcexcm+10 mm (strictly, the length required for reflection by the diffraction grating is included), and the mode distance xcex94xcex2 is about 0.03 nm.
Here, if the semiconductor laser device is made to operate by itself, the mode distance xcex94xcex1 is given by xcex94xcex1≈xcex2/(2neqxc2x7L1), wherein neq is the equivalent refractive index of a waveguide of the semiconductor laser device, xcex the central wavelength of gain thereof, and L1 the device length thereof. For example, when the emission wavelength xcex of the semiconductor laser device is 1485 nm, the equivalent refractive index neq of the waveguide is 3.4 and the device length L1 is 300 xcexcm, the mode distance xcex94xcex1 is about 1 nm, and when the device length L1 is 800 xcexcm, the mode distance xcex94xcex1 is about 0.4 nm.
When mode hop is caused by disturbance such as change in operating temperature or injected current, the central wavelength of emission first hops to an adjacent mode. Therefore, when the device length L1 of the semiconductor laser device is 300xcx9c800 xcexcm, mode hop happens at intervals of the mode distance xcex94xcex1 of at least 0.4 to 1 nm.
On the other hand, when the device length L1 of the semiconductor laser device is 800 xcexcm and the distance from the emitting surface of the semiconductor laser device to the equivalent reflecting surface of the diffraction grating is 10 mm or more, the mode distance xcex94xcex2 is about 0.03 nm or less, which is very small. Therefore, even if mode hop happens in the semiconductor laser device in the semiconductor laser module, the emission wavelength of the semiconductor laser module changes little, and therefore the output power thereof changes very little.
Therefore, when the semiconductor laser module of the present invention is to be used as a light source for EDFA pumping, multimode emission is more advantageous than single mode emission because change in output power is smaller in multimode emission than in single mode emission.
Further, the semiconductor laser device is arranged such that the difference between the central wavelength of the gain spectrum thereof and the central wavelength of reflection of the diffraction grating is xc2x120 nm or less, and that the half width at half maximum of the gain spectrum band of the semiconductor laser device is 10 nm or larger, whether the semiconductor laser device may have bulk structure or quantum well structure.
Since the semiconductor laser device thus arranged and the optical feedback medium having the diffraction grating are combined, the semiconductor laser module of the present invention has an emission spectrum showing higher wavelength stability for the following reasons:
For general explanation, an example of 1480 nm-band multimode emission semiconductor laser module in which a semiconductor laser device and a fiber bragg grating (optical feedback medium) are combined will be taken. In this example, let it be supposed that the central wavelength xcexLD of gain spectrum of the semiconductor laser device is 1465 nm, the central wavelength xcexFg of reflection of the fiber bragg grating (hereinafter referred to as xe2x80x9cFBGxe2x80x9d) is 1480 nm, the semiconductor laser device has Gaussian gain distribution, and a first multilayer coating has a fixed reflectance (for example, 1%) not depending on wavelengths.
In this case, there is a close relationship between the half width at half maximum of the gain spectrum band of the semiconductor laser device and the difference between the central wavelength xcexFg of reflection of the FBG and the central wavelength xcexLD of gain of the semiconductor laser device.
Here, the concept of pulling width will be explained.
In the above example, if the FBG is not provided, the emission wavelength of the semiconductor laser device is about 1465 nm. When the semiconductor laser device and the FGB are combined to form a compound resonator type laser, the relationship between the half width at half maximum of the gain spectrum and the pulling width is as shown in FIG. 1. Here, the compound resonator type laser has characteristic parameters as shown in FIG. 1. As indicated in FIG. 1, since the difference (xcexFgxe2x88x92xcexLD) between the central wavelength xcexFg of reflection of the FBG and the central wavelength xcexLD of gain of the semiconductor laser device is 15 nm, when the half width at half maximum of gain of the semiconductor device is about 23.5 nm or larger as read from graphs of xe2x80x9cshorter wavelength side pulling widthxe2x80x9d and xe2x80x9clonger wavelength side pulling widthxe2x80x9d, the emission wavelength of the compound resonator type laser is not 1465 nm, but close to 1480 nm which is the central wavelength of reflection of the FBG.
Such is the emission wavelength pulling effect caused by the FBG, and the difference (xcexFgxe2x88x92xcexLD) between the central wavelength xcexFg of reflection of the FBG and the central wavelength xcexLD of gain of the semiconductor laser device is usually called xe2x80x9cpulling widthsxe2x80x9d.
Next, the present invention""s principle as to how the emission wavelength of the compound resonator type laser formed by combining the semiconductor laser device and the FBG (optical feedback medium) is determined will be explained further in detail based on the relationship between gain and loss which is basic to the laser emission.
In order that the emission wavelength of the semiconductor laser device can be pulled into the central wavelength xcexFg of reflection of the FBG, it is necessary that a point of contact PC where a mirror loss curve CMR of the semiconductor laser module formed as the compound resonator type laser is in contact with a spectral curve CGS of net gain by certain injected current or carrier should always be at or near the central wavelength xcexFg of reflection of the FBG (depending on the half width at half maximum of reflection spectrum of the FBG, about xc2x12xcx9c3 nm or less) over the whole wavelength region of the semiconductor laser device.
Here, the gain of gain spectra in FIGS. 2, 4, 7 and 9 means so-called net gain G which is defined by the following expression:
G=xcex93xc3x97gxc3x97Lxe2x88x92xcex1ixc3x97L (dimensionless),
wherein xcex93 is a confinement coefficient of an active layer of the semiconductor laser device, g a gain coefficient (cmxe2x88x921) (it is to be noted that the half width at half maximum of gain spectrum as mentioned in claims refers to this g), L the device length of the semiconductor laser device (cm), and xcex1i an absorption coefficient of the semiconductor laser device (cmxe2x88x921) relative to the total loss.
For example, a semiconductor laser module comprising a semiconductor laser device and a FBG will be taken in which the half width at half maximum of gain spectrum of the semiconductor laser device is 30 nm, a dielectric multilayer coating, for example, of TiO2 and SiO2 showing a reflectance of 1% at the central wavelength of reflection is formed on the emitting surface of the semiconductor laser device, the central wavelength xcexLD of gain of the semiconductor laser device is 1466 nm (as shown in FIG. 2) when the emission wavelength of the whole semiconductor laser module is 1485 nm, and the central wavelength xcexFg of reflection of the FBG is 1485 nm.
Using this semiconductor laser module, the central wavelength (nm) of gain of the semiconductor laser device and the emission wavelength (nm) of the whole semiconductor laser module were measured, and it was found that when the central wavelength xcexLD of gain of the semiconductor laser device was within the range of xcex94A(1465.87 nm) to xcex94B(1504.34 nm), the emission wavelength of the whole module was close to 1485 nm, as shown in FIG. 3. Since the central wavelength xcexFg of reflection of the FBG is 1485 nm, it turns out that the pulling width at half maximum WHP is 19.13 nm (=1485xe2x88x921465.87) on the shorter wavelength side, and 19.34 nm (=1504.34xe2x88x921485) on the longer wavelength size.
As shown in FIG. 2, this semiconductor laser module satisfies, at the central wavelength xcexFg (=1485 nm) of reflection of the FBG, the emission condition Gxc3x97R=1 (wherein G is a power gain coefficient which represents net gain and R is a power absorption coefficient which represents mirror loss) which means that the mirror loss and the gain of the semiconductor device agree.
Therefore, in this semiconductor laser module, emission occurs in a manner that the central wavelength xcexLD(=1466 nm) of gain of the semiconductor laser device is pulled into the central wavelength xcexFg of reflection of the FBG.
If in FIG. 2, the central wavelength xcexLD of gain of the semiconductor laser device shifts beyond 1466 nm to the shorter wavelength side, the mirror loss curve CMR is no longer in contact with the spectral curve CGS of net gain at or near the central wavelength xcexFg of reflection of the FBG. In that case, no matter how current or carrier may be injected, the gain condition Gxc3x97R=1 can not be satisfied at the central wavelength xcexFg(=1485 nm) of reflection of the FBG, and therefore, in the semiconductor laser module, the central wavelength xcexLD of gain of the semiconductor laser device is not pulled into the central wavelength xcexFg (=1485 nm) of reflection of the FBG.
In that case, emission by the semiconductor laser module occurs not in a FBG emission mode but in an ordinary Fabry-Perot (FP) emission mode. FIG. 4 relates to such a case, in which the emission wavelength of the semiconductor laser device is 1465 nm. In FIGS. 2 and 4, power ratio curves CR0 (spectral distributions of relative power) are also shown. It is to be noted that the power ratio curve CR0 does not have the same form as the emission spectrum since in the power ratio curve, power ratios at close wavelengths are not separated.
FIGS. 5 and 6 show spectral distributions of actual output power (mW) of semiconductor laser modules corresponding to FIGS. 2 and 4, respectively.
FIG. 5 shows that the semiconductor laser module corresponding to FIG. 2 has most of its output power at the central wavelength xcexFg (=1485 nm) of reflection of the FBG. The semiconductor laser module has some output power near the central wavelength xcexLD (=1466 nm) of gain of the semiconductor laser device, because the reflecting mirror loss curve is close to the gain spectrum at the wavelength xcexLD.
On the other hand, FIG. 6 shows that emission by the semiconductor laser module corresponding to FIG. 4 occurs around the central wavelength xcexLD (=1466 nm) of gain of the semiconductor laser device. The semiconductor laser module has also some output power near the central wavelength xcexFg (=1485 nm) of reflection of the FBG, because the reflecting mirror loss curve is close to the gain spectrum at the wavelength xcexFg.
FIG. 7 shows a mirror loss curve CMR, a net gain spectral curve CGS and a power ratio curve CR0 of a semiconductor laser module comprising a semiconductor laser device and a FBG in which the half width at half maximum of gain spectrum of the semiconductor laser device is 30 nm, a dielectric multilayer coating of, for example, TiO2 and SiO2 showing a reflectance of 1% at the central wavelength of reflection is formed on the emitting surface of the semiconductor laser device, the central wavelength xcexLD of gain of the semiconductor laser device is 1479 nm when the emission wavelength of the whole semiconductor laser module is 1460 nm, and the central wavelength xcexFg of reflection of the FBG is 1460 nm.
Here, a point PC where the mirror loss curve CMR is in contact with the net gain spectral curve CGS is at the central wavelength xcexFg (=1460 nm) of reflection of the FBG, and the central wavelength xcexFg is on the shorter wavelength side of the central wavelength xcexLD (=1479 nm) of gain of the semiconductor laser device. In this semiconductor laser module, the central wavelength xcexLD of gain of the semiconductor laser device is pulled into the central wavelength xcexFg of reflection of the FBG.
As is clear from FIG. 8 which shows a relationship between the central wavelength (nm) of gain of the semiconductor laser device and the emission wavelength (nm) of the whole module, when the central wavelength xcexLD of gain of the semiconductor laser device is within the range of xcex94A(1440.63 nm) to xcex94A(1479.38 nm), the emission wavelength of the whole semiconductor laser module is pulled into the central wavelength xcexFg (=1460 nm) of reflection of the FBG.
On the other hand, FIG. 9 shows that when the central wavelength xcexLD of gain of the semiconductor laser device shifts beyond 1479.38 nm to the longer wavelength side, emission by the semiconductor laser module occurs mainly in a FP emission mode, not in a FBG emission mode.
For example, when the emission wavelength of the semiconductor laser device is 1480 nm, the central wavelength xcexLD of gain thereof is 1480 nm. Therefore, the mirror loss curve CMR is not in contact with the net gain spectral curve CGS at or near the central wavelength xcexFg (=1460 nm) of reflection of the FBG. Therefore, the semiconductor laser module satisfies the gain condition Gxc3x97R=1 not at the central wavelength xcexFg (=1460 nm) of reflection of the FBG but at the central wavelength xcexLD (=1480 nm) of gain of the semiconductor laser device, and therefore, the emission wavelength of the whole semiconductor laser module is not pulled into the central wavelength xcexFg (=1460 nm) of reflection of the FBG. FIGS. 10 and 11 show spectral distributions of actual output power (mW) of the semiconductor laser modules corresponding to FIGS. 7 and 9, respectively.
As is understood from FIGS. 2 to 11, in connection with increasing stability of emission wavelength, there is a very close relationship among the width of gain spectrum of the semiconductor laser device, a difference between the central wavelength xcexFg of reflection of the FBG and the central wavelength xcexLD of gain spectrum of the semiconductor laser device, and the pulling width at half maximum WHP.
When the semiconductor laser module comprising the semiconductor laser device and the optical feedback medium has the above described features, a large pulling width can be secured. Further, when the difference between the reflectance RGL of the diffraction grating at the central wavelength xcexFg of reflection and the reflectance R1 of the first multilayer coating formed on the emitting surface satisfies a condition RGLxe2x88x92R1xe2x89xa7xe2x88x922%, emission by the semiconductor laser module occurs mainly in a FBG emission mode, so that the output power is more stabilized.
Further, when optical coupling means is provided between the emitting surface of the semiconductor laser device and the optical feedback medium (waveguide or fiber having the optical grating therein) to have the power coupling efficiency of 50% or higher, the pulling width is prevented from narrowing, so that the semiconductor laser module is maintained as the compound resonator type laser module. It is to be noted that even if various parameters relating to the semiconductor laser module can be held at fixed values, a drop in the power coupling efficiency directly affects the wavelength pulling property.
In order that the wavelength pulling property may not drop beyond its limit, the semiconductor laser module needs to have the power coupling efficiency of 50% or higher. For example, the semiconductor laser module for which the parameters are determined as shown in FIG. 1 shows the optical coupling efficiency of 75%. If the power coupling efficiency is 100%, the pulling width at half maximum WHP is about 21.6 nm, and if the power coupling efficiency is 50%, the pulling width at half maximum WHP is about 16.6 nm. The difference is so large as about 5 nm.
In an ordinary pump laser for exciting an EDFA, output power is very important. It is said that in order to meet demands in various communication systems, an ordinary pump laser for exciting an EDFA needs to have output power of about 120 mW.
On the other hand, a laser device to be used in an ordinary semiconductor laser has a limit of output power. Further, a pump laser having a FBG has higher wavelength stability, but due to the attached FBG, a certain drop in output power as compared with a pump laser not having a FBG is inevitable. In order that a pump laser having a FBG may have the output power of 120 mW or higher, it is desirable that the power coupling efficiency is 50% or higher, provided that parameters for the pump laser are so determined that the pump laser can be manufactured easily.